This invention relates to pattern plates for printing, especially to such plates made by photoengraving wherein a metal plate is coated with a photoresist layer and exposed to light through a negative, after which the unexposed portion of the photoresist layer is removed and the metal is etched away, leaving the exposed resist-coated portions of the metal plate standing in relief. The metals are commonly magnesium, zinc and their alloys, and the etchant comprises aqueous nitric acid. In the early art, the tendency of the acid to undercut the resist during the etching step was combatted by "powdering" the plates after a shallow etch and repeating the operations until the desired depth was obtained.
Modern "powderless" etching was introduced by John A. Easley and Harry E. Swayze in U.S. Pat. Nos. 2,640,763 to 2,640,767, inclusive, (1953) and further improved subsequently, e.g. in U.S. Pat. Nos. 2,763,536; 2,828,194; 2,940,836; 2,979,387; 3,023,138; 3,152,083; 3,239,466; 3,251,777; 3,320,171; 3,330,765; 3,337,462; 3,357,921; 3,490,968; 3,510,372; 3,725,158; 3,725,159; 3,728,180; 3,730,899; 3,736,197; and 3,922,229 and others relating to the formulation of etchant compositions for improved etching of metal photoengraving plates.
For use of pattern plates in printing, especially for the purpose of making therefrom a stereotype pulp mat and thence casting a stereotype metal plate for letterpress printing, the pattern plates are preferably etched to a depth of about 30 mils (0.030 inch) in the larger "open" areas between print lines, i.e. in the "white" areas between the inked lines of the print. Accordingly, it has become commonplace to use stock engraving metal sheets that are about 40 mils thick in order to avoid risk of etching through the plate.
With the increasing costs of metal and the increased concern for waste disposal and conservation of material resources, it would be desirable to reduce the thickness of etchable metal in pattern plates for use in printing arts.
However, the fundamental requirements of a pattern plate include structural integrity and dimensional stability. The usual structure of a pattern plate is characterized by a multiplicity of areas, often individually tiny areas, of the sheet corresponding to the relief printing surfaces or image areas of the pattern plate, between which are the etched-out areas corresponding to the non-printing or non-image areas of the pattern plate. It is essential that the treatment and process to form the pattern plate not cause holes therethrough and not cause weakening that could result in loss of part of the printing portion of the pattern plate. It is also essential that the pattern plate be dimensionally stable so that the printing or image areas of the plate are held in fixed relationship despite the presence of relatively thin areas in the non-printing, non-imaged portions of the pattern plate and the use of very heavy pressures in the step of making mats from such pattern plates.
The present invention provides pattern plates having structural integrity and dimensional stability wherein the metal portion consists entirely of etched magnesium of minimal thickness usually corresponding to the maximum depth of the etch.